For Obama, the debates are a chance to halt McCain's momentum and re-establish his image as a fresh political force after eight years of Republican White House control.
They give McCain the opportunity to press doubts about Obama's experience and try to show he's still on the top of his game at age 72.
In past debate settings, McCain has managed to win over crowds with his humor and empathy, and at the same time, slice up foes with a flash of contempt.
Obama is known as a powerful speaker. He has shown wit and charm in political debates, but can come across as aloof, windy and somewhat lifeless. One debate expert says the key for Obama will be to stay on offense to shape the discussion and connect with undecided voters.
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